1. Field of the Invention
The present invention generally relates to a thin film transistor (TFT) and a TFT array substrate, and more particularly to a TFT and a TFT array substrate using a copper signal line and a method of fabricating the same.
2. Discussion of the Related Art
A flat panel display device, for example, a plasma display panel (PDP) device, a liquid crystal display (LCD) device and an organic light emitting diode (OLED) device, is widely developed and used. Since the flat panel display device has advantages in thickness, weight and power consumption, it is used as a display device instead of the cathode ray tube (CRT).
To meet requirements for a flat panel display device having a large size and high resolution, a width of a signal line is desired to be decreased and a length of the signal line is desired to be increased, which causes line resistance to rapidly increase and incur a voltage drop.
To reduce the line resistance, a relative low resistance material, e.g., copper (Cu) and silver (Ag), is used for the signal line, e.g., a gate line. Particularly, a patterning process for Cu is easier than that for Ag, and a specific resistance, i.e., 2.1˜2.3 μΩcm, of a Cu film is smaller than a specific resistance, i.e., 3.1 μΩcm, of an aluminum (Al) film, which is widely used as the signal line. In addition, the Cu layer has stronger resistance against a hillock problem than the Al layer. Accordingly, the Cu film is in the spotlight as a material for a signal line.
However, Cu has a bad adhesive property with glass. Accordingly, a peeling problem of a Cu line from a glass substrate may occur. In addition, since a size of Cu ion is small, the Cu ion is easily diffused through a silicon oxide film. Accordingly, in the instance of forming an insulating layer of silicon oxide on a Cu line, a Cu ion of the Cu line is diffused into the insulating layer such that an insulating property of the insulating layer is degraded.
Moreover, since Cu has high oxidization property, Cu is easily oxidized when being exposed to air. The Oxidized Cu increases resistance and stress of the Cu line such that electrical properties of the Cu line are degraded.